refactor(api): split monolithic handlers.go into domain-specific files (task 3)

Split internal/api/handlers.go (1045 lines) to improve KISS/SRP:
- handlers_auth.go    – bootstrap, login, logout, health, session cookies
- handlers_media.go    – sets, media CRUD, tags, favorites, notes, progress
- handlers_share.go    – create/list/revoke shares, share page, share stream
- handlers_admin.go    – trash, rescan, users, permissions
- handlers_file.go     – stream, download, thumbnail, regenerate thumbnail

Shared helpers (writeJSON, readJSON, pathID, serveFileResult,
mimeTypeForFilename, etc.) remain in handlers.go.

All tests pass: go test ./... -race -cover.

0 files changed, 0 insertions, 0 deletions